Any geotechnical engineer – Dirt Doctor to some – worth his weight in soil could have predicted what was going to happen to a building near a four-story deep excavation. This was an excavation at the University of British Columbia Okanagan campus in Kelowna, B.C. That’s about 40 feet deep – a big hole!
The excavation was dug for construction of a 43 story high-rise and four-level underground parkade. Eighty-four (84) tenants were evacuated from one building near the excavation, the Hadgraft Wilson Place because of potential collapse. Problems developed over months as the excavation got deeper including bricks falling from nearby buildings onto the sidewalk below.
I’ve consulted on big failures – a railway embankment in northern Australia that almost took a train down – but nothing like this. You can see a picture of the excavation and read about it in the following link. And also get a feel for the depth of the excavation knowing that the orange security fence surrounding the excavation is about the height of a tall construction worker.
Google .https://www.cbc.ca/news/canada/british-columbia/kelowna-apartment-evacuation-1.7160537..)
This failure is big but the cause is simple. The sides of all excavations in soil slump down or cave in a little or a lot, or something in between depending on the type of soil. An excavation in stiff clay would slump down a little whereas loose sand and gravel a lot. Part of the excavation for this building caved in a lot according to news reports and a picture at the link above.
The surface of the soil beyond the excavation settles and moves when this happens. Depending on the magnitude of the settlement, cracks may appear in structures like buildings, parking lots or roads supported on the soil behind the excavation.
This settling and cracking is sometimes okay – when it’s really really tiny – other times it’s not. When it’s not okay the side of the excavation is shored or propped up. Something like a wall is built against the side of the excavation – a shoring-wall like mentioned in the news.
The shoring-wall is built against the vertical side of the excavation in the soil to hold it in place. The shoring is more or less elaborate and strong depending on the depth of the excavation and the nature of the soil.
Like all materials used in construction the shoring gives a little – moves – when there is pressure on it. The pressure comes from the side of the excavation wanting to fall or slump down.
The surface of the soil in back of the excavation settles or subsides a little when the pressurized shoring-wall moves a little. The foundations of a building in this soil settles too and cracks appear in the building – like did in the buildings near the excavation in Kelowna as reported in the news.
The amount of foundation settlement depends on factors like the following:
- The depth of the excavation,
- Nature of the soil,
- Strength of the shoring,
- Distance of the building foundations from the face of the excavation,
- Pressure from the foundations
Anything on the surface of the soil in back of an excavation settles and subsides, a little or a lot or something in between.
Geotechnical engineers know this based on common knowledge, some knowledge of the soil in the area from a published surficial geology map, and the depth of the excavation – they wouldn’t need to leave their office. Look at me way Down East telling you this.
The magnitude of the building settlement and cracking, and whether or not it would get reported in the newspapers, is another matter. This is another level of engineering that would require me, like any engineer, to get out of the office. Get on site and get my hands dirty and mud on my boots.
But knowing why is easy. Also knowing that something like this would happen – before the excavation was dug – is easy too. And that a shoring-wall would need to be properly designed and constructed.
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This settling and cracking-up occurred in the building where I took my lectures in civil engineering. I remember a 2.5 inch crack in the corner of our lecture room. It was still there years later during a class reunion.
An addition to the building was being constructed at the time that involved an excavation adjacent to the foundations of our lecture room. The excavation was shored up but not well enough – I remember a wall of vertical, steel I-beams and horizontal wood planks. The pressure on the shoring-wall caused it to move and soil in back to settle undermining the foundations of the room. I don’t think there were any Dirt Doctors on site in this case.
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(Posted by Eric E. Jorden, M.Sc., P.Eng. Consulting Professional Engineer and Forensic Engineer, Geotechnology Ltd., Halifax, Nova Scotia, Canada, April 29, 2024 ejorden@eastlink.ca)